The invention relates to polymeric particles having a coating which provides improved anti-blocking properties, and to a method of manufacturing the coated polymeric particles. The invention also encompasses equipment used in manufacturing the coated polymeric particles.
A variety of polymeric materials are produced from an initial reaction sequence at elevated temperatures and pressures. The resulting polymeric materials exist in a molten form. The polymeric materials may be further processed directly from the molten phase, or be cooled to ambient temperature prior to further processing. Depending on processing conditions and composition of the reactants, the cooled polymeric materials may be converted to pellets or other physical forms which may be easily packaged or may be first cooled and then ground, chopped or otherwise processed prior to further processing of the material. Some polymeric materials, by the nature of their composition, tend to exhibit cold-flow properties. Examples of such polymeric materials are ethylene vinyl acetate, very low density polyethylene (i.e., polyethylene with a density of 0.90 grams per cubic centimeter or less), ethylene methyl acrylate, and ethylene n-butyl acrylate. These materials, even though previously processed to produce particulate units, such as pellets, chips or powders, will nonetheless tend to flow at ambient temperatures and pressures. The particles, after cold-flow, tend to agglomerate to form a single mass which is difficult to reinstitute into the component pellets, granules or the like. It is highly preferred that polymeric materials which exhibit these cold-flow tendencies continue to have free-flowing characteristics.
Various attempts have been made to provide a surface coating to polymeric particles to limit or eliminate the tendency to agglomerate. To this end, materials such as bisoleamide have been incorporated into the polymer reaction mixture prior to formation of the polymer particle. The bisoleamide is not miscible with the polymeric material and, in time, migrates to the surface of the particle to provide a coating which resists agglomeration. It has also been known to coat silica and talc powders onto the surface of a polymer particle to inhibit agglomeration.
It is also known to apply a micro-fine polyolefin powder coating onto polymeric pellets to decrease the tackiness of the pellets. The micro-fine powder can be applied by incorporating the powder into the chilled water of an underwater pelletizing device which cools the pellets cut after extrusion. Alternatively, the micro-fine powder can be coated onto formed polymer pellets by tumbling, airveying or the like. Also, the coating may be applied by electrostatically charging a bath of fluidized powder with an electrical potential different from that of the polymer pellets.
In the preparation of polymeric particles having improved anti-agglomeration properties, there remains a need for a product which not only exhibits superior anti-agglomeration properties following treatment with a coating material, but also retains this property after packaging, transport, and in further processing.
To provide superior anti-agglomeration, or tackiness-reducing, properties, one aspect of the invention is directed to coated polymeric particles, with each of the coated particles comprised of a polymeric substrate particle and a block-reducing coating on the surface of the substrate particle. The coating includes polymeric coating particles which advantageously may be in the form of a micro-fine powder. Also disclosed herein is a process for applying the polymeric coating particles onto the polymeric substrate particles, as well as equipment which is especially useful in producing the coated polymeric particles.
The coated polymeric particles may be produced by incorporating the polymeric coating particles into an aqueous coating composition and applying the composition to the surface of the substrate particles such as by spraying. The resulting coated particles then may be subjected to a drying step, such as in a fluidized bed, to remove substantially all traces of water and other volatile components from the coated particles.
The method of forming the coated particles may include introducing polymeric substrate particles into an auger assembly which conveys the substrate particles through a spray zone to an outlet zone. During operation of the auger assembly, the substrate particles are sprayed with an aqueous coating composition as the particles are mechanically mixed while being conveyed forward. From the outlet zone, the resulting coated particles may be conveyed to a drying apparatus such as a fluidized bed. The fluidized bed operates at elevated temperatures and provides an air flow which thoroughly agitates the coated particles. This combination of elevated temperature and agitating air flow removes water and other volatile components. The coated particles thereafter are cooled and transferred to a packaging or storage station. Preferably, the coated particles are cooled in a downstream section of the fluidized bed prior to transfer to the packaging or storage station.
If desired, the auger assembly may include a screw mounted inside a chamber, preferably such as a barrel, wherein the outer diameter of the screw is slightly less than the inside diameter of the barrel to thereby provide minimal clearance between the crest(s) of the screw helical flight(s) and the interior wall of the barrel. This arrangement substantially prevents backflow of particles toward the inlet of the auger assembly. If desired, the screw diameter and screw pitch may be uniform along the length of the screw, although this is not required. The pitch is the length of a longitudinal section of the shaft of the screw occupied by a flight, a flight being any full, 360 degree rotation of the spiral-like portion of the screw which extends radially outward from the shaft of the screw. At least one spray head is mounted to introduce aqueous coating composition into the barrel. In another aspect of the invention, the barrel of the auger is fitted with at least two spray heads for introducing the aqueous coating composition containing the polymeric coating particles into the barrel and into contact with the polymeric substrate particles. In an embodiment of the auger assembly where two or more spray heads are employed, the screw has a uniform pitch along its length, and the spray heads are positioned in a straight line on the barrel parallel with the longitudinal axis of the screw and of the barrel, the distance along the longitudinal axis of the barrel between spray heads is advantageously a non-integral multiple of the pitch of the screw. With the spray heads configured in this way, the flow of aqueous coating composition through the spray heads into the barrel is never completely interrupted during rotation of the screw.
In a further aspect of the invention, the spray heads are mounted so that the orifice of each spray head nozzle is substantially flush with the interior cylindrical surface of the barrel. In this fashion, the movement of crests of the screw, the particles, or both, across the flush-mounted spray heads minimizes the formation of coating composition solids build-up inside the auger assembly. As a result, auger assembly downtime is minimized and the substrate particles are more uniformly coated with the desired concentration of polymeric coating particles.